Recombination-like events between virus and host genomes occur in cells lytically-infected with the DNA tumor viruses, SV40 and polyoma. The product of the recombination-like event is a closed-circular viral DNA molecule in which extensive portions of the viral genome have been deleted and replaced by covalently-linked host DNA sequences (termed substituted virus DNA). The objectives of the proposed work are (a) to clarify the process which generates substituted virus DNA, (b) to determine the relationship between this process and the virus-induced neoplastic transformation of cells, and (c) to investigate the possible activities of substituted tumor viruses as mutagens and transducers of cellular genetic determinants.